Continuous vacuum dryers designed with rotating shelves have been available on the market for several decades. Krauss Maffei of Germany markets a continuous vacuum dryer. This device has a plurality of plates mounted on a vertical shaft. Wet product is fed from a feeder unit which may be under vacuum or pressure onto the uppermost plate. Product is plowed across the plate by a rotating transport arm. The product is plowed across the plate by the rotating transport arm in the form of a ridge extending toward an outer edge of the plate. The product falls onto the next plate below. The foregoing process is repeated as many times as there are plates. Dry product is discharged from the plate dryer. This device has limitations in that it is large in size. Heat transfer, efficiency, ease of cleaning and troubleshooting are limited due to size and slow operational speeds. Clean-up before changing products can be a major cost. Furthermore, the cost of maintaining non-atmospheric pressure such as vacuum can be substantial due to the multiple shelves and large seals required to accomplish this job. Products having a high moisture content such as slurries cannot be efficiently dried.
U.S. Pat. No. 3,897,218 discloses a polycondensation reactor mounted within a horizontal drum. A plurality of partitions divide a series of drums which are mounted within the outer drum which are rotated by a shaft. The partitions define the level of liquid in each drum. The rate of flow of product between drums is controlled by the partitions. The speed of the shaft is not varied to control a rate of flow between the drums or a dwell time within each drum.